The subject invention relates to a press felt for use in papermaking, cellulose and similar machines and particularly to a press felt provided with a loop-type seam by means of which the ends of the flat woven felt are joined together in the papermaking machine to form an endless cloth. The invention also concerns a method of manufacturing and installing press felts of the kind referred to.
Papermaking machines comprise three main sections, usually referred to as the forming section, the press section and the dryer section. To the forming section is supplied a fibrous suspension often comprising less than 1% fibres which are to be formed into a paper sheet on the forming fabric. When the sheet thus formed leaves the forming section is has a moisture content of between 70 and 80 percent, depending on the type of papermaking machine used.
Following the forming section the thus formed sheet 6 is passed through the press section wherein the sheet is advanced on a press felt in through the press nip between the press rolls, where water is removed from the sheet. The press section may include several press nips and it is important that a maximum amount of the remaining water is removed in this section, as this allows a reduction of the energy consumption in the ensuing drying process. When the sheet leaves the press section its moisture contents usually are reduced to values below 60 percent.
From the press section the sheet is transferred to the dryer section wherein moisture is transferred from the sheet through heat. This reduces the moisture contents in the sheet to values below 10 percent, in many cases down to approximately 5 percent.
Like other papermaking cloths press felts have undergone remarkable developments during the past twenty years. It is characteristic of the modern press felt that it comprises a soft part which is positioned closest to the paper web and serves to protect the latter, and an incompressible part designed to receive water from the paper sheet and carry it away from the press nip. In addition, the demands on felt evenness are considerable since the smallest irregularity or unevenness necessarily leads to damages to the paper sheet. Further, felt unevenness may be the cause of vibration generated upon passage of the felt through the press nip. As an example of the magnitude of the pressure applied could be mentioned that the normal press felt thickness of between 3 to 4 millimeters is reduced in the press nip to about 1.5 millimeters.
The requirements that press felts, as explained above, thus have to meet have led to the general opinion among experts in the field that only felts that are produced in endless form, that is felts woven in tubular form or joined felts, may be used. Seams of the kind used in dryer felts or dryer fabrics have been considered inevitably to damage and even to cause sheet breaks. The high speeds, up to approximately 1000 m/min, at which the felt and the sheet operate in many of up-to-date papermaking machines, have been considered to cause vibration upon passage of the seams of the kind referred to through the press nip. Although in U.S. Pat. Nos. 2,883,734 and 2,907,093 are disclosed press felts incorporating a loop seam these prior-art press felts are not known ever to have been used in practice. Based on the knowledge of the prior-art technology and on experience from the development work that has led to the subject invention the inventors consider it to be evident that loop seams used in conventionally woven, single-layer felts and wherein no protection is provided for the seam loops, are predestined to failure.
Despite the suggestions in the two U.S. patent specifications referred to it has hitherto been taken for granted that the only viable possibility is to use press felts which are woven in tubular form or are joined to endless form. Without exception, all press sections to date are constructed with expensive means and systems allowing the press rolls in the press section to be raised to allow new felts to be installed. The incompressibility of the press felts increases the rigidity of the press felts, which makes their installation and mounting more difficult.